1. Field of the Invention
The present invention relates to processes for making polydiorganosiloxane based graft copolymers, and more particularly relates to a process for making said graft copolymers by first preparing cross-linked rubber particles from dihydroxyfunctional polydiorganosiloxane and then grafting vinyl monomer thereon.
2. Description of the Related Art
Processes for producing polyorganosiloxane based graft copolymers by (a) homogenizing organosiloxane to form a latex and polycondensating in latex form the organosiloxane with an organosiloxane graft-linking agent, and (b) graft polymerizing a vinyl monomer onto the resulting polyorganosiloxane is known, see European Patent Application 0166900 which is incorporated herein by reference. Such a process however utilized a polyorganosiloxane consisting essentially of 90 to 99.999 mole % of an organosiloxane monomer having a unit represented by the general formula R.sub.y.sup.7 SiO.sub.(4-y)/2 wherein R.sup.7 denotes hydrogen, methyl, ethyl, propyl, or phenyl, and y denotes a number of 0, 1, or 2. Evidence has been presented in prior articles indicating that such monomer can migrate through the water phase to new polymerization sites, see J. C. Saam and D. H. Huebner, Abstracts 3.sup.rd International Conference, Surface and Colloid Science, Stockholm, 1979,166 (Saum et al) and Emulsion Polymerized Polydimethylsiloxane, Daniel Graiver, David J. Huebner and John C. Saam, (Graiver et al) presented at a meeting of the Rubber Division, American Chemical Society, Toronto, Ontario, Canada, May 10-12, 1983, Rubber Chemistry and Technology, Volume 56, pages 918 to 926. Monomer migration into the water phase in emulsion polymerization following homogenization can result in the formation of smaller particles during polymerization than were formed during homogenization thereby effectively making particle size control difficult.
Emulsion polymerization of hydroxy terminated oligomers of polydimethylsiloxane (MW,Mn2600) has been taught with the recognized effect of there being no evidence of migration or changes in emulsion particle size when oligomer is used instead of monomer, see Graiver et al above. Additionally, a method of producing aqueous latex of crosslinked polydiorganosiloxane has been taught, see Bauman et al, U.S. Pat. No. 4,618,645 which is incorporated herein by reference. These two references however do not teach to make graft copolymers using the resultant emulsion particles and do not teach using crosslinked polydiorganosiloxane rubber particles obtained from a process involving homogenization and polymerization of dihydroxy polydiorganosiloxane polymers as the rubber particles for graft copolymer.
Additionally it is known to adjust homogenization pressure to vary particle size, see Saam et al (J. Poly. Sci.:Poly. Chem. Ed., vol.20,3351-3368(1982), Rubber Chemistry and Technology, vol.56, no.5,918-26(1983)) wherein the effect of changing homogenizer pressure on the particle size variation was studied, but that reference does not teach to adjust viscosity of dihydroxy functional polydiorganosiloxanes to influence the particle size distribution of silicone emulsions and does not teach the use of these emulsions with controlled particle size to make impact modifiers for thermoplastic.